Free fatty acids (FFA) mobilized from adipose tissue stores represent a major fuel source during prolonged exercise in humans. The aims of the studies in this proposal are 1) to evaluate the relative contributions of adipose tissue lipolysis and FFA reesterification to FFA availability during exercise, 2) to examine the nutritional and hormonal factors which regulate FFA mobilization in exercising men and women of average physical fitness, and 3) to assess the mechanisms by which changes in FFA availability affect glucose homeostasis during exercise in humans. Specifically, these studies will address 1) the potential gender differences in fat mobilization during equivalent exercise, 2) the influence of changes in dietary fat on FFA availability, 3) the effects of obesity and body fat distribution on fat mobilization during exercise, and 4) the relative roles of insulin, glucagon, and catecholamines in the regulation of FFA availability in exercising men and women. All subjects will be studied under basal postabsorptive conditions, during 2h of submaximal exercise on a cycle ergometer, and 90 min of post-exercise recovery. Isotopic tracer methods ([2H5] glycerol, [1-14C] palmitate, [3- 3H] glucose) will be utilized to trace lipolysis, FFA turnover, and glucose kinetics. Rates of FFA and glucose oxidation will be assessed by indirect calorimetry. Rates of FFA reesterification will be calculated from the rates of lipolysis, FFA turnover, and FFA oxidation. The pancreatic/pituitary clamp and glucose clamp techniques will be utilized in these studies to examine the isolated effects of the exercise-induced changes in insulin, glucagon, and catecholamines on FFA mobilization and glucose metabolism during exercise. These studies will be the first to address the potential gender differences in the in vivo lipolytic response to exercise and the nutritional and hormonal regulation of lipolysis and FFA reesterification during exercise in humans. A better understanding of the factors that influence fat mobilization during exercise will be relevant to the clinical application of exercise in the treatment of obesity, diabetes mellitus, and insulin-resistant states.